Jealious of ex after miscarriage

My partner and I have 3 kids between us. But 2 are his and 1 is mine. We both have our kids full time. My daughter has never meet her biological dad so my partner is her dad. My partners kids see their mum during work hours 4 days a week. Its so nice having them all with us at night. We have a beautiful little mixed up family. I was so happy so content. Until I feel pregnant and had a miscarriage. Our baby wasn't planed we weren't ready for another baby yet we didnt even know we were pregnant until it was to late.
But now every thing has changed. I feel different. I've become jealous of his ex that she has kids to him and I dont. I look at their baby photos and it hurts. But it never use to. She had his babies and I lost ours. Is this a part of grieving? Will it go away. I feel like I've let him down that I can't be the mother of his kids but his ex can who he isnt with but I am. I still love our 3 kids more than the world but I want to feel like I did before before I lost our baby. Has any one been through a similar situation?